Place versus direction with two-way prepositions

These two-way prepositions fall into two groups based on their static case:

• Accusative (Direction) / Locative (Location): na (on/onto), v (in/into).
• Accusative (Direction) / Instrumental (Location): nad (above/over), pod (under), před (in front of), za (behind), mezi (between/among).

Important Nuances:

• Motion vs. Destination: A common myth is that verbs of motion always take the accusative. This is false. The accusative is triggered by the destination. If you are running around under a bridge, you use the static case (Běhám pod mostem). You only use the accusative if you run under the bridge from the outside (Běžím pod most).
• Vocalization: Prepositions ending in a consonant (like v, pod, před) often add an -e (e.g., ve vodě) to make pronunciation easier before certain consonant clusters. For the prepositions that take the instrumental case, this vocalization is mandatory before the pronoun mnou (me) because it starts with a consonant cluster (m+n), making the extra vowel necessary to avoid a tongue-twister (e.g., přede mnou, pode mnou, nade mnou). Prepositions already ending in a vowel remain unchanged (e.g., za mnou, mezi mnou a tebou).
• The "Into" Exception & Pairing Rule: While v + Accusative technically means "into", modern Czech overwhelmingly prefers do + genitiv for physical motion into a place (though v + Accusative is still strictly required for certain abstract or fixed expressions, like věřit v Boha or vstoupit v platnost). To choose the right destination preposition, remember this pairing rule: If the location uses v (in), the destination uses do + Genitive (e.g., jsem v Praze → jedu do Prahy). If the location uses na (at/on), the destination uses na + Accusative (e.g., jsem na poště → jdu na poštu).
• Three-Way Prepositions: Be aware that za can also act as a three-way preposition, taking the genitive case to express time or circumstance (e.g., za války - during the war).